How do you name alkenes with double bonds? How do you name alkynes with two triple bonds? See all questions in Naming Alkanes, Alkenes, and Alkynes. Impact of this question views around the world. You can reuse this answer Creative Commons License. Watch this brief video tutorial to review the nomenclature process.
Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes.
Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties.
The differing geometries are responsible for the different properties of unsaturated versus saturated fats. Ethene, C 2 H 4 , is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene propylene Figure 6 ; the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as a new form of isomerism.
Figure 6. Expanded structures, ball-and-stick structures, and space-filling models for the alkenes ethene, propene, and 1-butene are shown. Ethylene the common industrial name for ethene is a basic raw material in the production of polyethylene and other important compounds. Over million tons of ethylene were produced worldwide in for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.
Polymers can be natural starch is a polymer of sugar residues and proteins are polymers of amino acids or synthetic [like polyethylene, polyvinyl chloride PVC , and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties see Chemistry in Everyday Life: Kevlar, later in this chapter.
An example of a polymerization reaction is shown in Figure 7. The monomer ethylene C 2 H 4 is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of —CH 2 — units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging bags and films. Figure 7. The reaction for the polymerization of ethylene to polyethylene is shown.
Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling.
Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.
Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate soft drink bottles can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene bags and polypropylene cups, plastic food containers , can be recycled or reprocessed to be used again.
Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code see Figure 8. These operations have been in effect since the s and have made the production of some plastics among the most efficient industrial operations today. Figure 8. Each type of recyclable plastic is imprinted with a code for easy identification. The name of an alkene is derived from the name of the alkane with the same number of carbon atoms.
The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene. The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond:. Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs.
As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms. The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid.
This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. The 2-butene isomer in which the two methyl groups are on the same side is called a cis -isomer; the one in which the two methyl groups are on opposite sides is called a trans -isomer Figure 9. The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:.
Figure 9. These molecular models show the structural and geometric isomers of butene. This reaction is called an addition reaction. The hybridization of the carbon atoms in the double bond in an alkene changes from sp 2 to sp 3 during an addition reaction.
For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:. The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene. We begin counting at the end of the chain closest to the double bond—in this case, from the left—the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond—and they are on the same side of the double bond—this molecule is the cis- isomer, making the name of the starting alkene cis pentene.
The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond:. This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.
Hydrocarbon molecules with one or more triple bonds are called alkynes ; they make up another series of unsaturated hydrocarbons. The simplest member of the alkyne series is ethyne, C 2 H 2 , commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:. The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain.
Chemically, the alkynes are similar to the alkenes. The reaction of acetylene with bromine is a typical example:. The first six alkanes are tabulated below:. All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulae that you can draw for each molecular formula. Cycloalkanes also only contain carbon-hydrogen bonds and carbon-carbon single bonds, but the carbon atoms are joined in a ring.
The smallest cycloalkane is cyclopropane. By joining the carbon atoms in a ring, two hydrogen atoms are lost. The general formula for a cycloalkane is C n H 2n and these are non-planar molecules with the exception of cyclopentane and exist as "puckered rings".
This is known as the "chair" form of cyclohexane because of its shape, which vaguely resembles a chair. Cyclohexane also has a boat configuration not shown. The boiling points shown are for the "straight chain" isomers in which there are more than one Figure 1. Notice that the first four alkanes are gases at room temperature, and solids do not start to appear until about C 17 H The temperatures cannot be more precise than those given in this chart because each isomer has a different melting and boiling point.
By the time you get 17 carbons into an alkane, there are unbelievable numbers of isomers! Cycloalkanes have boiling points that are approximately 10 - 20 K higher than the corresponding straight chain alkane.
There electronegativity difference between carbon and hydrogen 2. These forces will be very small for a molecule like methane but will increase as the size of the molecules increase. Therefore, the boiling points of the alkanes increase with the molecular size. Regarding isomers, the more branched the chain, the lower the boiling point tends to be.
Van der Waals dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules with substantial amounts of branching to lie close together compact compared with long, thin molecules.
The slightly higher boiling points for the cycloalkanes are presumably because the molecules can get closer together because the ring structure makes them tidier and less "wriggly"! Alkanes both normal and cycloalkanes are virtually insoluble in water but dissolve in organic solvents.
The liquid alkanes are good solvents for many other covalent compounds. When a molecular substance dissolves in water, the following must occur:. Breaking either of these attractions requires energy, although the amount of energy required to break the Van der Waals dispersion forces in a compound, such as methane, is relatively negligible; this is not true of the hydrogen bonds in water.
Methane through Butane are very flammable gases at standard temperature and pressure STP. Longer alkanes are waxy solids; candle wax generally has between C 20 and C 25 chains. As chain length increases ultimately we reach polyethylene, which consists of carbon chains of indefinite length, which is generally a hard white solid. Alkanes react only very poorly with ionic or other polar substances.
The pKa values of all alkanes are above 50, and so they are practically inert to acids and bases. In crude oil the alkane molecules have remained chemically unchanged for millions of years.
Reaction with oxygen leads to combustion without any smoke; with halogens, substitution. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes.
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